J. Phys. Chem. C 2010, 114, 4725–4732
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Synthesis of Pt@Cu Core-Shell Nanoparticles by Galvanic Displacement of Cu by Pt4+ Ions and Their Application as Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells A. Sarkar and A. Manthiram* Electrochemical Energy Laboratory, Materials Science and Engineering Program, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: January 24, 2010
Carbon-supported Pt@Cu “core-shell” nanoparticles with Pt-Cu alloy core and Pt shell have been synthesized by a galvanic displacement of Cu by Pt4+ at ambient conditions, followed by a leaching out of unreacted Cu on the surface by treating with 9 M H2SO4. X-ray diffraction (XRD) data indicate the formation of a Pt-Cu alloy below the Pt shell. Energy dispersive spectroscopic (EDS) analysis in a scanning electron microscope (SEM) reveals that the experimental Cu content is much lower than the initial nominal Cu content, confirming the displacement of a significant amount of Cu by Pt. X-ray photoelectron spectroscopic (XPS) studies indicate surface enrichment by Pt. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements demonstrate an enhanced catalytic activity for the oxygen reduction reaction (ORR) for optimum Pt@Cu compositions compared to that found with commercial Pt catalyst, both per unit mass of Pt and per unit active surface area basis. Moreover, the surface area specific activities of the Pt@Cu samples increase linearly with increasing initial nominal Cu content. The increase in activity for ORR is ascribed to an electronic modification of the outer Pt shell by the Pt-Cu core. 1. Introduction Proton exchange membrane fuel cells (PEMFC) employing hydrogen as a fuel cell provide an efficient and clean alternative to the presently used internal combustion (IC) engines. PEMFC converts the chemical energy of hydrogen fuel directly into electrical energy, thereby offering high efficiency with little pollution. The operating characteristics of PEMFC including the low temperature of operation are particularly attractive for transportation applications. Considerable worldwide research is being undertaken for the commercialization of the PEMFC technology. However, the cost and durability issues are major barriers for large-scale manufacturing and deployment of PEMFC. Additionally, the kinetically sluggish reaction at the cathode, where oxygen is reduced to water by addition of protons and electrons, limits the efficient utilization of the fuel. The principal component that determines the oxygen reduction reaction (ORR) kinetics is the cathode catalyst.1-6 Platinum and platinum alloys are widely used as cathode electrocatalysts for ORR in PEMFC. Even on high surface area Pt catalyst, the cathode overpotential is several millivolts.2 Evidently, for PEMFC to be viable and competitive to IC engines, significant improvement in the activity toward ORR is required. Alloying of Pt with other elements like V, Cr, Fe, Co, Ni, Cu, and Ir has been shown to improve the kinetics of ORR significantly while reducing the cost as well.7-13 The enhanced activity of the alloys for ORR has been attributed to lattice contraction that facilitates the adsorption of molecular oxygen, formation of Pt skin upon dissolution of the transition metal atoms in acidic solution, and d band occupancy effects.14-19 Recently, a novel class of electrocatalysts has been developed by selective voltammetric dealloying of Cu from Pt-Cu * To whom correspondence should be addressed. Fax: (1) 512 471-7681. E-mail:
[email protected].
alloys.13,20,21 The dealloying of Cu from the carbon-supported Pt-Cu alloy electrocatalyst was carried out by subjecting the Pt-Cu alloy to potential cycling repeatedly to dissolve the less noble Cu from the catalyst surface. Moreover, it had been postulated that dissolution of Cu from Pt-Cu alloy at high potential leads to a “core-shell” structure consisting of a Pt-Cu alloy core encapsulated by a Pt shell. The enhancement in activity for ORR has been attributed to geometric effects where a favorable Pt-Pt bond distance facilitates the electroreduction of oxygen. Another novel class of “core-shell” electrocatalysts was obtained by Cu underpotential deposition (UPD) on various noble metals and subsequent galvanic replacement of the Cu atoms by Pt4+ ions. This technique resulted in the formation of a single layer of Pt atoms on other noble metal surfaces such as Pd, Ru, and Au, resulting in a multifold increase in activity based on per unit mass of platinum.22-25 We present here the synthesis and characterization of a series of carbon-supported Pt@Cu “core-shell” nanoparticles with a Cu or Pt-Cu alloy core and Pt shell by a galvanic displacement reaction between Pt4+ and Cu. The synthesis scheme adopted here essentially combines the volatmmetric dealloying feature with the galvanic displacement to achieve a structure analogous to that obtained by voltammetric dealloying of alloyed Pt-Cu nanoparticles. Galvanic displacement reactions has been extensively used for the synthesis of noble metal core-shell nanoparticles, nanocubes, nanocages, and tetrahedrons, using silver nanoparticles as a sacrificial template.26-30 Recently, hollow Pt spheres synthesized by galvanic displacement of Ag by Pt4+ have been found to offer enhanced activity for oxygen reduction reaction.31 However, because of the tendency of copper to form oxides in neutral or basic media, there are only a very few reports on the galvanic displacement of Cu by noble metals ions.32 Although Pt@Cu “core-shell” nanoparticles synthesized by a sequential polyol reduction of Cu2+ and Pt4+
10.1021/jp908933r 2010 American Chemical Society Published on Web 02/24/2010
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J. Phys. Chem. C, Vol. 114, No. 10, 2010
ions have shown greater selectivity and activity for NOx reduction33 and galvanic displacement of electrochemically deposited Cu by Pt has been reported,34 to the best of our knowledge, this is the first report on the synthesis and characterization of Pt@Cu core-shell nanoparticles in aqueous media by a galvanic displacement reaction between Pt4+ and Cu and their exploration as cathode electrocatalysts in a fuel cell. The characterization of the Pt@Cu core-shell samples by X-ray diffraction (XRD), energy dispersive spectroscopic (EDS) analysis in a scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), cyclic voltammetry (CV), and hydrodynamic polarization measurements for ORR in rotating disk electrodes (RDE) is presented. 2. Experimental Section 2.1. Chemicals. Copper sulfate (CuSO4 · 5H2O) (Fisher Scientific) and chloroplatinic acid (H2PtCl6 · 6H2O) (Strem Chemicals Inc.) were used as the precursors. Trisodium citrate (C6H5Na3O7 · 2H2O) (Fluka) and citric acid (C6H8O7) (Alfa Aesar) were used as stabilizers and to prepare buffer solutions. Sodium borohydride (NaBH4) (Fisher Scientific) was used as a reducing agent. Vulcan XC72R (Cabot) was used as the carbon support. All the chemicals were used as-received. 2.2. Synthesis of Copper Nanoparticles and Pt@Cu “Core-Shell” Nanoparticles. The synthesis of Cu nanoparticles was achieved by a reduction of CuSO4 · 5H2O with NaBH4 in a buffered aqueous medium at room temperature. Briefly, 250 mL of a buffer solution with pH 3.0-6.0 and a buffer strength of 0.2 M was prepared by adding the required amounts of sodium citrate and citric acid in deionized water. The concentrations of citric acid and sodium citrate were evaluated by using the Henderson-Hasselbalch equation and the pKa values for citric acid (3.13, 4.76, and 6.40).35 The pH of the buffer was measured with a Corning 315 pH meter and was found to be within (0.05 of the intended pH value. To determine the precise pH condition for the reduction of Cu and formation of Cu nanoparticles, 200 mg of CuSO4 · 5H2O was dissolved in 250 mL of the buffer solution of various pH values (3.0-6.0) and purged with N2 gas for 30 min. A 500 mg sample of NaBH4 dissolved in 50 mL of water was then added dropwise and the reaction was continued for 2 h under constant stirring and nitrogen purging to reduce the Cu2+ ions to Cu. The reaction mixture was then centrifuged, the clear colorless supernatant liquid was discarded, and the precipitate was collected. On the basis of the XRD data of the precipitate, the optimum pH condition to obtain Cu was determined to be 3.0. Additionally, for a comparison, another sample was prepared in a manner as described above, but without pH control. For the synthesis of Pt@Cu “core-shell” nanoparticles, 250 mL of the buffer solution with a pH of 3 and a buffer strength of 0.2 M was prepared by the method described earlier. Subsequently, 160 mg of Vulcan carbon and the required amount of CuSO4 · 5H2O were added into the buffer solution to give x wt % Cu in carbon (20 e x e 140). The solution was then purged with N2 gas for 0.5 h under constant stirring, 500 mg of NaBH4 dissolved in 50 mL of water was added dropwise, and the reaction was conducted for 4 h under continuous stirring and N2 purging to reduce the Cu2+ ions to Cu. After 4 h of reaction, 106.2 mg of H2PtCl6 · 6H2O (corresponding to 40 mg of Pt metal on 160 mg of carbon or 20 wt % Pt on 80 wt % C) dissolved in 50 mL of water was added dropwise, and the galvanic displacement reaction was conducted for 2 h. The carbon-supported Pt@Cu electrocatalysts thus obtained were
Sarkar and Manthiram filtered, washed repeatedly with deionized water, and dried overnight in an air oven at 80 °C. A 100 mg sample of the powder thus obtained was digested with 50 mL of 9 M H2SO4 for 3 h to remove any unreacted copper as well as any oxides formed due to the handling of the samples in ambient. Finally, the mixture was filtered and washed repeatedly with deionized water. The electrocatalysts thus obtained with x wt % Cu in carbon are hereafter designated as Pt@Cu x%. To make it clear, all the Pt@Cu x% samples had 20 wt % Pt in 80 wt % C while considering only Pt and C, but additionally had varying amounts of Cu. In other words, the total metal (Pt + Cu) loading in the samples is higher than 20 wt % and the total carbon content is accordingly lower than 80 wt % depending on the Cu content in the final product. 2.3. Instrumentation and Measurements. The samples were all characterized by XRD with Cu KR radiation. XRD patterns were recorded with a counting time of 12 s per 0.02° between 30° and 90°. All the XRD patterns were fitted by a mix of Gaussian and Lorentzian profiles (50% Gaussian) after background subtraction, using the Jade MDI software. For most of the samples, the lattice parameters were evaluated using the first three reflections between 2θ ) 30° and 85°. The Cu:Pt ratios in the synthesized samples were assessed by averaging the ratios obtained at four different spots in the EDS analysis with a JEOL-JSM5610 SEM having an Oxford instruments EDS attachment. XPS characterizations were performed with a Kratos Axis Ultra DLD spectrometer, using monochromatic Al KR radiation. To avoid the effect of charging on the binding energy values, an automatic charge neutralization system was used. The near surface compositions in the samples were obtained by integrating the peak intensities corresponding to the Pt 4f and Cu 2p regions of the spectra. Morphological and particle distribution studies were carried out with a JEOL 2010F highresolution transmission electron microscope (TEM) operated at 200 keV. CV characterizations were carried out with a standard single compartment three electrode cell having a Pt mesh counter electrode, a glassy carbon (5 mm diameter) working electrode, and a double junction Ag/AgCl reference electrode, employing a Autolab PGSTAT302N (Eco Chemie B.V., Netherlands). All potentials are, however, reported against normal hydrogen electrode (NHE). In a typical experiment, 2 mg of the carbonsupported catalyst was ultrasonicated in 1 mL of deionized water and 1 mL of 0.15 wt % Nafion solution (diluted from 5 wt % Nafion solution obtained from Electrochem Inc. by adding the appropriate amount of ethanol) until a dark homogeneous dispersion was formed. Twenty microliters of the aliquot was drop casted onto the glassy carbon electrode (5 mm in diameter, Pine Instruments, U.S.A.) to give an effective carbon-supported catalyst loading of 20.37 µg Pt/cm2 for 20 wt % Pt on carbon. Before each experiment, the glassy carbon electrode was polished to a mirror-like finish with 0.05 µm alumina (Buehler). The CV experiments were conducted in N2 purged 0.5 M H2SO4 (solutions prepared from Fisher Scientific high purity Optima grade 18 M H2SO4) at a scan rate of 50 mV/s between 0.0 and 1.0 V (vs NHE) at ambient conditions. The stable voltammograms obtained after the initial cycling (50 cycles) are reported here. Thereafter, rotating disk electrode (RDE) experiments were conducted with the same catalyst coated glassy carbon disk electrode (5 mm diameter) mounted onto an interchangeable RDE holder (Pine Instruments, USA) in O2 saturated 0.5 M H2SO4. The rotation rate was kept at 1600 rpm and initially the potential was cycled between 0.0 and 1.0 V (vs NHE) at 20 mV/s for 5 cycles. Following this, the potential was scanned
Synthesis of Pt@Cu Core-Shell Nanoparticles
Figure 1. XRD patterns of (a) Cu nanoparticles prepared at various pH values and without pH control and (b) Pt@Cu x% (40 e x e 140) samples. The dashed line (---) and the dotted line ( · · · ) refer respectively to the expected positions of Cu (111) and Pt (111) reflections. The reflections marked with b refer to Cu2O.
linearly from 0.0 to 1.0 V (vs NHE) at 5 mV/s and the polarization curves thus obtained were used for comparison. 3. Results and Discussion 3.1. Synthesis. The propensity of Cu to oxidation (both bulk and surface) is a significant barrier to the synthesis of Cu nanoparticles and subsequent galvanic displacement by Pt4+. Although Cu/Cu2+ redox couple (E0 ) +0.340 V vs NHE) is placed positive to the H2/H+ redox couple in the electrochemical series and copper inherits some nobility of its group (IB of the periodic table), Cu nanoparticles formed by reduction of Cu2+ are susceptible to oxidation due to the alkalinity of the NaBH4 solution. As evident from the XRD patterns shown in Figure 1a, by decreasing the pH to 3.0, it was possible to obtain purephase metallic Cu without any bulk oxide impurity. In addition, capping of Cu nanoparticles by citrate ions inhibits the particle growth as is apparent from the broadening of the (111) reflection in all the Cu samples prepared in the buffered solution compared to the samples synthesized without any buffer. After the synthesis of the Cu nanoparticles supported on Vulcan carbon, the second step involved galvanic displacement of Cu by Pt4+ according to the reaction 2Cu + [PtCl6]2- f Pt + 2Cu2+ + 6Cl- . The difference between the E0 values of Pt/ [PtCl6]2- (0.742 V vs NHE) and Cu/Cu2+ (0.340 V vs NHE)35 is adequate for the displacement reaction to be both thermo-
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4727 dynamically as well as kinetically favorable. However, the stoichiometry of the reaction and the difference in the atomic radius values of Cu and Pt (rCu ) 1.28 Å and rPt ) 1.39 Å) suggest that displacement of Cu atoms on the outermost layer by Pt4+ would cause only a fraction of the top layer of the nanoparticles to be covered by Pt atoms, thus leaving the inner layers for further reaction. This would result in the formation of a large number of steps and kinks on the outermost shell of Pt@Cu nanoparticles. Additionally, as the reaction proceeds, a concentration gradient of Pt is likely to develop with Pt on the outermost shell and a Pt-Cu alloy of varying composition beneath the outermost shell. It is also likely that both the displacement reaction and the nucleation and growth of Pt layers would initiate on low coordination sites of the Cu nanoparticles. Also, the Pt atoms resulting from the galvanic displacement reaction would be nonepitaxial because of the large difference between the atomic radii and lattice parameter values of Cu and Pt.27 It should be noted that in an analogous situation, room temperature alloy formation has been observed during the galvanic displacement of nanoporous Cu by Pd2+ and with Ag@Au “core-shell” nanoparticles.36,37 With an increase in initial Cu content, keeping the reaction conditions the same, both the dispersion (number density of Cu nanoparticles) and the particle size are expected to increase. This would influence the surface sensitive displacement reaction as an increase in dispersion would increase the surface area of the Cu nanoparticles while an increase in particle size would decrease the surface area. In addition, as the number of Pt atoms participating in the reaction is kept constant, an increase in both particle size and dispersion would influence the Pt-Cu alloy composition beneath the Pt shell. Especially, an increase in particle size would increase the concentration of Cu in the Pt-Cu layer. Additionally, with much larger particles, a small Cu core might be present beneath the Pt-Cu alloy layer. Depending on the initial concentration and sizes of the Cu nanoparticles, there might be incomplete Pt coverage as all Cu atoms on the surface might not be displaced by the Pt4+ ions. Thus, the final acid wash step would remove the surface Cu atoms and the unreacted Cu nanoparticles, facilitating the formation of a pure Pt shell over a Pt-Cu core. 3.2. Structural and Compositional Characterizations. 3.2.1. XRD, EDS, and XPS Analysis. Figure 1b presents the XRD patterns of Pt@Cu x% samples obtained after the acid treatment in 9 M H2SO4. Interestingly, all the samples show evidence of an alloyed Pt-Cu structure instead of either pure Pt or Cu as is apparent from the large shift in the (111) reflection toward higher angle compared to the (111) reflection of pure Pt. Indeed, evaluation of the lattice parameter values for all the samples (except Pt@Cu 20% and Pt@Cu 40% samples as the XRD patterns of these samples were too diffuse to be amenable to analysis) as shown in Table 1 and Figure 1b suggests alloying between Pt and Cu or a substitution of smaller Cu for larger Pt. The lattice parameter values decrease with increasing initial nominal Cu content due to an increasing final Cu content in the sample (Table 1) and an increasing degree of alloying of Pt with Cu. Moreover, the Cu:Pt atom ratio in the “core-shell” nanoparticles as determined from SEM-EDS analysis and the lattice parameter values using Vegard’s law (Table 1) agree closely with each other for samples with low initial nominal copper content (
